Restriction endonucleases and hybridization-mapping techniques have been used to prepare detailed physical maps showing the genome organization of human adenoviruses. The overall homology of viral DNA from different serological subgroups is only about 25% (by EM heteroduplex mapping, filter hybridization, and common restriction sites), yet the complex organization of early and late gene clusters in Ad 2 DNA (subgroup C) and Ad 7 DNA (subgroup B) is virtually identical. Thus overall genome organization does not appear to determine the variety of biochemical and biological features which distinguish human adenovirus subgroups. Defective particles of Ad 7 are produced in abundance in infected cells. Preparations based on the buoyant density of particles with different DNA size classes (about 10% intervals) have been characterized by restriction endonuclease mapping and electron microscopy of the DNA. The particles contain selectively viral DNA sequences, in various lengths proceeding from the molecular left end of the Ad 7 genome. The pattern is suggestive of abortive assembly of a significantly larger pool of intermediates than is found in cells infected with subgroups C adenoviruses. The most abundant incomplete, or defective particles contain from 10 - 15% of the Ad 7 DNA and show transformation activity in hamster cell cultures. Experiments are now in progress to assess the expression of the single early gene cluster in the purified particles of defective Ad 7 by hybridization-mapping of the RNA from cells infected with these particles. Studies related to Ad 7 virion assembly and incomplete particles are underway, including in vitro association of Ad 7 empty particles and viral DNA. BIBLIOGRAPHIC REFERENCE: Pettersson, U., C. Tibbetts and L. Philipson, 1976. Hybridization Maps of Early and Late Messenger RNA Sequences on the Adenovirus Type 2 Genome. J. Mol. Biol. 101: 479-501.